Venezuelan Equine Encephalitis (VEE) is a mosquito-borne viral disease which primarily affects horses and can also infect humans. It has been classified as endemic in certain regions of Central and South America. VEE can affect all age groups, with no differences shown across genders. Unfortunately, there is no specific treatment or vaccine yet for the family of encephalitic arboviruses such as VEE. Treatment is either supportive or palliative. While most issues that occur during the course of infection do not persist after the end of infection, neurological sequelae can persist. Due to the limited capabilities in medicine to treat the disease, it is crucial that we find ways of preventing infection and the spread of VEE in the first place. Make sure to read on as this article will provide you with all you need to know about VEE. 

Introduction

The virus is a single-stranded RNA alphavirus which are solely mosquito-borne pathogens. There are multiple species of mosquitos that are capable of transmitting VEE, however, the Culex and Aedes genera has been shown to be the primary vector (vehicle by which the virus is transmitted).¹

Due to the virus’ mosquito-borne nature, it is priority number one to control these two genera of mosquitos to prevent the sporadic spread of disease. There are various modes of mosquito control such as insecticide resistance, chemical population control, natural products and biological control.² Humans can also spread the virus but do not contribute significantly. On the other hand, horse infections are the primary amplifying hosts for the virus (the multiplication and increased concentration of the pathogen). 

Understanding VEE

Virus characteristics

Alphaviruses such as VEE are significant emerging mosquito-borne pathogens which are found globally and known to cause endemics and epidemics.³ For the purpose of this article, we will focus on situations arising specifically in Central and South America. Alphavirus replication is a very well-researched topic, although less is known about how the VEE virus specifically replicates. Like most RNA viruses, alphaviruses replicate inside the cytoplasm of the host.³

Transmission and lifecycle

As mentioned, the Culex and Aedes species of mosquitos are the primary vectors for VEE. The mosquitos themselves become infected when feeding on viraemic hosts (such as horses that carry the virus in their bloodstream) and consequently, the virus begins to replicate in the mosquitos. In a chain-like reaction, the infected mosquitos begin to feed on other horses where the virus is able to amplify itself and thus replicate in higher numbers the more horses are infected.⁴

Though horses have been shown to be the primary hosts, humans can of course become infected in an incidental capacity. On the bright side, humans are considered “dead-end hosts” and therefore cannot contribute significantly to the transmission cycle (cannot pass from person to person).⁴

Impact on public health and agriculture 

Culex and aedes species

In Latin America, these species of mosquitos are considered to be among the most medically significant vectors in urban environments. The Culex genera is responsible for nasty diseases such as West Nile Virus and Lymphatic Filariasis. The Aedes species, on the other hand, is linked to the transmission of diseases such as Dengue Fever and Zika Virus.⁵

As far as the impact on agriculture, the Culex species can transmit viruses to birds which is detrimental to poultry farming and bird populations. Additionally, the Aedes species is responsible for yellow fever which, although is primarily a human health concern, can impact wildlife and ecosystems.⁵

Distribution and habitats 

The predominant habitats of these species of mosquitos can range from urban to domestic environments, just as long as humidity is present. Culex species tend to thrive in places such as sewage systems, ponds, or overgrown vegetation. While Aedes species are more predisposed to breed in domestic environments such as flowerpots or water containers. This makes the Aedes species more dangerous to humans due to their domestic nature and their increased likelihood of spreading disease.⁶

Across Latin America, there are various locations which are important breeding grounds for mosquitos. Breeding grounds are especially significant in this part of the world due to the vastness of diverse habitats. When speaking of their distribution, there is an emphasis on Northern areas of South America such as the Caribbean Islands and the Amazon.² Climate and seasonal factors are crucial in determining when mosquito distribution is likely to increase. Rainy seasons and warm temperatures have been observed to create favourable conditions for them and thus increase VEE virus replication.     

Control strategies for VEE-transmitting mosquitos 

Importance of mosquito control 

Mosquito control of the Culex and Aedes genera is the first step in attempting to prevent VEE transmission. VEE can cause severe illness in humans, with symptoms ranging from flu-like to severe neurological complications. Additionally, the health of horses can be significantly impacted and cause big losses in the agricultural sector. 

Mosquito control often begins with integrated pest management such as targeted insecticide or holistic approaches to reduce the possibility of epidemic outbreaks.⁷

Role of mosquito control in reducing disease transmission 

The World Health Organisation (WHO) recommends Integrated Vector Management (IVM) to manage mosquito populations. 

The IVM program comprises various features such as:

• Method selection based on local vector biology knowledge
• Utilisation of biological, and chemical control measures, and community mobilisation (often combined)
• Collaboration within the health sector and involvement of other public and private sectors
• Rational use of insecticides.⁸

Mosquito control is an essential component of public health strategies aimed at preventing and controlling mosquito-borne diseases such as VEE. Disrupting the mosquito-borne transmission cycle in turn minimises mosquito contact between humans. These methods contribute significantly to reducing the impact of VEE on communities. 

Benefits of the IVM programme

As opposed to other insecticide-related strategies for mosquito control, the IVM programme takes a more holistic approach by emphasising environmentally friendly and sustainable methods. IVM programs are specifically designed using data from surveillance and monitoring of mosquito populations allowing for targeted interventions.⁹

One major benefit of IVM programmes is their ability to adapt to local environmental and epidemiological conditions. This allows for localised mosquito control in the various environments of Latin America. This is crucial in the case of VEE as Latin America is incredibly diverse and mosquito control strategies need to be tailored to each individual ecosystem to allow for enhanced outcomes and reduced transmission. 

Specific methods for mosquito control

Surveillance and monitoring

In order to understand the gravity of VEE transmission it is important to analyse the source of transmission. Therefore, regular surveillance of mosquito populations is essential to understand the relevance of disease. Monitoring these populations permits experts to assess and implement control measures. 

Larval mosquito control

The objective of this method is to manage larval populations before adulthood. However, this type of monitoring can only be successful if mosquito breeding sites are identified and accessible ^[9]. In the case where breeding sites are accessible, the sites can be modified or even eliminated. This includes removing standing water, treating water bodies with larvicides, and implementing changes to reduce potential breeding habitats.

Although in theory, this would seem like an incredibly effective way of reducing disease transmission, in reality, larval control may not be enough to attain low enough levels of mosquito populations.⁹

Adult mosquito control

As larvicide treatments are not adequate enough to lower vector populations significantly, an adult mosquito control program can be implemented to complement larval control. Pesticides aimed at adult mosquitos can be applied from hand-held devices, trucks, or aircraft. The ability to apply pesticides from aircraft greatly helps efforts to reduce disease transmission quickly in the event of an outbreak, as has been done before.⁹

In the case of the Culex genera, aerial spraying has been described as being particularly effective. According to various studies, they require multiple, closely timed treatments. Pesticides are applied with the use of ultra-low-volume (ULV) technology where a very small volume of pesticide is used per acre in an aerosol format.

IVM limitations

As you can imagine, the IVM strategy requires certain standards of infrastructure, funding, and trained personnel. In areas of the world such as South America, aggressively implementing IVM is not always possible due to an adverse socio-economic background. In order for IVM to function successfully, deficiencies in all areas of public health would need to be correctly addressed. Furthermore, there would need to be a strengthened collaboration with local and international academic and scientific institutions.¹⁰ 

Summary

In conclusion, there are various ways to manage mosquito control with the goal of reducing VEE virus transmission. However, there are certain considerations which need to be addressed regarding the area infrastructure. It is common knowledge that the Culex and Aedes genera are major virus-transmitting species of mosquitos and dangerous to humans. Breeding sites need to be identified in order to distribute the correct amount of pesticide treatment without too much waste. With the right humanitarian aid, it could be possible to one day rid these areas of disease.